On a longer telephoto, say a 300mm on a full-frame (35mm) you would need 1/300 sec.

Image stabilization

Camera (and lens) makers are now adding image-stabilization to their lenses, which lowers the shutter speed needed. Generally the makers will rate the level of stabilization in stops. Keep in mind these ratings are used for marketing and may be a bit inflated, but I am going to do my calculations based on the numbers being correct to keep it simple.

If you are using a 100mm lens with a 2 stop image stabilization system on a APS-C cropped sensor then:

I'd suggest (a) clarifying that it's just a rule of thumb, not a guarantee and (b) that manufacturer claims on IS systems are often highly optimistic.
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ReidJul 30 '10 at 21:01

I would add to what Reid says and say that it also depends on you and how steady your hands are. If you take multiple shots (assuming your subject doesn't move!), you can sometimes get a sharp shot at longer shutter speeds.
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Jared UpdikeJul 30 '10 at 22:05

Thanks for the suggestions! I agree that this 'rule' is just a starting point. If you have steady hands you can probably push it a bit, but knowing where to start is half the battle.
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chills42♦Jul 31 '10 at 1:44

Okay, mine is not a technical answer, but I think it has some merit that the technical answers lack: empiricism. Try using different speeds and see what you can hand-hold.

For each lens (and zoom setting, if applicable), handhold the camera while on shutter priority,and see what the slowest shutter speed YOU can use is without shaking the camera. Different people have hands that shake to different degrees.

By the way, I would check out whether or not it is blurry on a monitor, not on the camera LCD. You just can't see clearly enough to be sure whether there is blur on the LCD (unless you have a much better quality LCD screen than I've seen.

Most cameras allow you to magnify the image you're viewing, so it's usually possible to check for pixel-level blur even without a computer.
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cheJul 31 '10 at 6:06

1

well said, Rabbi. And it doesn't just depend on the person, but the photographer's health, state of mind, physical comfort, environmental conditions, etc. I've shot in a force 10 winter gale at times, being battered by 120kmh winds is a good way to be unstable (with or without tripod) even if you're shielded from the worst of the winds by things like buildings.
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jwentingDec 14 '11 at 7:16

You can determine the minimum shutter speed to avoid camera shake by
1) applying the following approximate rules of thumb. (See Wikipedia article - rule of thumb)
2) or carrying out careful measurements, as I did.

Note that these are approximate rules and are heavily dependent on photographer technique, which is why they are called rules of thumb (my thumb and your thumb are not the same).

b) With image stabilisation.
Here the rule of thumb is to take the above calculation and increase the above shutter speeds by either two or three stops, depending on your confidence in the manufacturer.

The graph below shows the main result of this study. With a 50mm lens motion blur was kept below one pixel down to a shutter speed of 1/8 sec, which is more than acceptable.
This is effectively equivalent to the following rule (for the Pentax K7):

min shutter speed = 1/(focal_length*1.6) - 3 stops.

However at shutter speed below about 1/30 sec the result are critically dependent on photographer technique. The graph below shows how variability of the results increases rapidly at lower shutter speeds, which illustrates the importance of photographer technique.

A couple of answers have already mentioned the 1/FL rule of thumb. Keep in mind, however, that this is only a rule of thumb, not an iron-clad law. Depending on how steady you are, you may find that you can (or must) adjust it.

Good technique is critical here. The same techniques used by target rifle shooters work nicely. First, get the steadiest stance you can: prone is best, kneeling second best, standing up your last choice. If you have to shoot standing, put your left hand directly under the lens and brace your elbow against your chest if possible (especially important with longer/heavier lenses). Take a fairly deep breath, then let it about halfway out before you squeeze the shutter release.

Narrowing the field of view is magnifying the final image. It doesn't matter where the step occurs. Think about a P&S with 6mm lens.
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EruditassJul 30 '10 at 19:30

3

The object on the APS-C shot will be larger relative to the overal frame due to the sensor-cropping, regardless of pixel densities and photosite size. If you print them both to a 8x12, an object will be larger on the APS-C shot.
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EruditassJul 30 '10 at 19:38

3

If you crop an image, you are increasing the magnification as you are zooming in on the image. The shake-induced blur will be magnified as well and the guideline, which is based on not cropping, applies less. The APS-C and cropped FF will look the same. Of course, as you crop, it will typically be obscured by pixel size, as larger format cameras tend to have lower pixel densities.
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EruditassJul 30 '10 at 19:59

1

@John: I believe you are thinking in terms of raw pixels and numbers, but we are projecting all of the pixels onto a common image physical image size in inches.
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EruditassJul 30 '10 at 22:36

1

The 1/focal length guide is an ESTIMATE! I can hand hold a little below the 1/FL rule on my crop sensor camera. So, for anyone who's confused by the crop sensor part, I'd say just ignore it. If you have a 200mm lens, start with the assumption that 1/200 is as slow as you should go. 100mm lens = 1/100. And anything less than 1/60 is usually not handholdable. Then experiment on your own and find out what works (reliably) for YOU; some people are more steady than others and as you learn better holding techniques you'll find you handhold at lower speeds than even the 1/FL rule on a crop camera.
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Erica MarshallJul 31 '10 at 5:07